1. Field of the Invention
This invention concerns novel insecticidal compounds isolated from specialized cells of the avocado. In particular, this invention concerns avocadofurans identified as 2-(tetradecyl)furan, 2-(pentadecyl)furan, 2-(hexadecyl)furan, 2-(heptadecyl)furan, 2-(octadecyl)furan, 2-(1EZ-pentadecenyl)furan, 2-(8Z,11Z-heptadecadienyl)furan, (8Z,11Z-heptadecadienyl)furan and the triglyceride triolein present in the avocado oil extracted from the avocado idioblast cells. These compounds inhibit larval growth and lengthen the larval development of insect pest and are highly synergistic, with a combination of any two compounds showing much greater toxicity than the same amount of any one alone.
2. Background Art and Related Disclosures
Agriculture of many states and countries is severely damaged by the crop pests. For example, in California, the beet armyworm, Spodoptera exigua is a key pest on lettuce and tomatoes. This pest is resistant to chlorinated hydrocarbons, cyclodienes, organophosphates and pyrethroids and has also demonstrated potential for resistance to the carbamate methomyl which is a predominant chemical used for its control. Moreover, even if effective, these chemicals are potentially hazardous to humans and animals and lately there has been pressure from public and from regulatory agencies to avoid using this kind of chemical insecticides.
Therefore, it would be advantageous to provide an efficient biocontrol of these type of pest using nontoxic insecticides.
Avocados, Persea americana Mill (Lauraceae), are oleaginous fruit in which oil levels in the mesocarp, or flesh, vary from 1-2% of fresh weight early in the season, to over 30% late in the season. The oils are valuable nutritionally as a source of energy, vitamins, and unsaturated lipids, with the edible portion of the fruit being rich in oleic, palmitic, linoleic, and palmitoleic acids as described in The Biochemistry of Fruits and Their Products: The Avocado Pear, Vol. 2: 1-63, A. C. Hulme (Ed.), Academic Press, London.
Several compounds isolated from avocado fruit have been reported to have various types of biological activity. For example, 1-acetoxy-2-hydroxy-4-oxo-heneicosa-(12Z,15Z)-diene (persin) extracted from the leaves of fresh avocado inhibited growth of 4th instar silkworm Bombyx mori L. larvae (Agr. Biol. Chem., 39:1167 (1975), and Jap. J. Appl. Entomol. Zool., 20:87 (1976)) at a concentration of 200 .mu.g/g within 2 days. In 1982, persin was isolated from peels of unripe avocado fruit and found to possess an antifungal activity against Colletotrichum gloeosporioides Penz Phytopat., 72:1578 (1982). Later on, a related compound, 1-acetoxy-2,4-dihydroxy-n-heptadec-16-ene, isolated from the peel and flesh of unripe avocado fruit, was also found to be active as a fungicide (J. Phytopathol., 132:319 (1991)).
A further study identified a series of related compounds with antifungal activity, including 1,2,4-trihydroxyheptadec-16-yne, 1,2,4-trihydroxy-n-heptadec-16-ene, and 1-acetoxy-2,4-dihydroxyheptadec-16-yne, as described in Phytochemistry, 31:93 (1992).
Avocado leaves, seeds, roots and fruit were shown to contain specialized idioblast oil cells scattered throughout the avocado mesocarp, composing approximately 2% of the tissue volume. Moreover, these cells have been reported to contain an oil that differs from other lipids found in the fruit mesocarp.
Recent studies, described in Physiol. Mol. Plant Pathol., 43:319 (1993) demonstrated the antifungal activity of two compounds present in these idioblast oil cells to the fungus, C. gloeosporioides.
Natural compounds containing furan rings are commonly found in many plant species. However, except for the specific phytochemicals such as furanocoumarins, few studies examined their biological role. A particular group of these furan derived plant compounds is present in avocados, Persea americana Mill (Kauraceae) and related plants. These compounds are commonly referred to as avocadofurans and were first isolated, identified and described in Tetrahedron, 25:4617 (1969). Two avocadofurans, namely 2-(trideca-12-ynyl)furan and 2-(trideca-12-enyl)furan, were isolated from avocado fruit and seeds. Later, other avocadofurans were isolated from seed extracts.
Although as compounds many avocadofurans were identified, their biological activity remained unexplored. The first report on some antibacterial activity appeared in 1970 when the growth inhibitory activity of 2-(trideca-12-enyl)furan against Bacillus subtilis and Staphylococcus aureus was reported.
Avocadofurans isolated from avocado leaves were also tested for their insecticidal activity against Bombyx mori L. However, their activity was very small, if any.
It has now been discovered that the fractions isolated from specific idioblast cells of avocado are potent nontoxic insecticides of insect pests.
It is, therefore, a primary subject of the invention to provide a new nontoxic insecticides which would be effective against common crop insect pest. The insecticides of the invention are avocadofurans having significant inhibitory bioactivity on the growth and development of the insect, particularly insect of generalists herbivore of S. exigua type.
All patents, patent applications and publications cited herein are hereby incorporated by reference.